The scores on an Economics examination are normally distribu

The scores on an Economics examination are normally distributed with a mean of 74 and a standard deviation of 11. If the instructor assigns a grade of A to 10% of the class, what is the lowest score (rounded to the nearest integer) a student may have and still obtain an A?

Solution

P(X>c)=0.1

--> P((X-mean)/s <(c-74)/11) =1-0.1=0.9

--> P(Z<(c-74)/11) =0.9

--> (c-74)/11 = 1.28 (from standard normal table)

So c= 74+1.28*11=88